A tunable laser source (sometimes referred to as TLS) may encounter a mode hop phenomenon (mode hop sometimes is referred to as mode hop wavelength error), whereby as the wavelength of the light (electromagnetic energy) output from the laser is swept over a range of wavelengths, the light may skip from one wavelength to another wavelength rather than relatively smoothly transitioning (or sweeping) over an intended range of wavelengths.
An ideal sweep of a TLS is illustrated in FIG. 1. Referring to FIG. 1, a linear correlation is shown between the beginning wavelength and end wavelength of the TLS as plotted over increasing time. The dotted lines identify ranges wherein the mode hop phenomenon may be detected, i.e., in a region that is displaced or offset from the generally linear correlation between the wavelength of the TLS and time. When the wavelength is greater than the correlated wavelength (e.g., above the linear correlation), such a phenomenon is referred to as a “forward mode hop”. Likewise, when the wavelength is less than the correlated wavelength (e.g., below the linear correlation), such a phenomenon is referred to as a “backwards mode hop”.
Mode hop may occur in a laser due to mode competition in the laser and represents a shift from one mode of the laser to another; and with such a mode hop shift, a shift in wavelength of the output light from the laser may occur. The mode hop may be in either direction, e.g., increasing or decreasing wavelength. In some lasers the shift in wavelength may occur between about 18 pm (picometers) and about 40 pm; the shift may occur additionally or alternatively between other wavelengths or wavelength ranges—these examples are not intended to be limiting.
The occurrence of mode hop leads to wavelength error in the characterizing of an optical component being tested or measured in an optical component measurement system, for it may be expected that an optical component is being illuminated by incident light of a given wavelength, but the wavelength actually is different from that expected. The mode hop error in the TLS, for example, may be in effect a discontinuity in the sweep of the wavelength of the incident light to the device under test (DUT) and, thus, would cause error in characterizing the DUT. Thus, the occurrence of mode hop can detrimentally affect measurements made by optical measurement instruments and the characterizing of the optical component measured by such instrument.
Optical components are used in telecommunication systems and in other systems. Some examples of optical components include optical fibers, lenses, filters, wavelength division multiplexers, splitters, fiber bragg gratings and other devices. The requirements for accuracy of the wavelength performance of optical components continue to increase, for example, as bandwidth, signal speed, number if signals transmitted, etc., increase. Thus, the performance accuracy of the optical components necessarily increases. Signal loss, polarization dependency, optical interference, and cross-talk between signals are examples of factors that must be reduced as the number of signals, wavelengths, and frequencies increase and extend over wider bandwidths. One approach to increase the number of signals carried in an optical telecommunication system, for example, uses wave division multiplexing (WDM). WDM and other techniques used to increase signal, data, information, etc. transmission or carrying capability, accordingly, increase the wavelength accuracy requirements or characteristics and other optical characteristics required of the optical components used. Correspondingly, there is a need to increase the accuracy and capabilities of optical measurement systems used to characterize or to test such optical components, e.g., to measure or to determine characteristics such as signal loss, wavelength shifts, optical interference, and cross-talk between signals.
Some optical measurement instruments use a tunable electromagnetic energy source, such as, e.g., a TLS, or other device to provide input electromagnetic energy at selected wavelengths or over a range of wavelengths to a device under test (DUT), such as, e.g., an optical component, optical device, optical system, or the like, (these terms, the terms “device under test” or “DUT”, etc., may be used equivalently herein) for testing by such instrument. Electromagnetic energy from the DUT can be measured and correlated with the wavelength and/or other characteristics of the incident electromagnetic energy expected from the source. The results of such measurements, correlation, and the like may be used to characterize the DUT. It is desirable that optical measurement systems for characterizing DUT's be accurate.
For brevity of the description herein, the DUT sometimes will be referred to as an optical component, which may be, e.g., an optical fiber, filters, wavelength division multiplexers, splitters, fiber bragg gratings, or other device that is intended for use in an optical system, such as a telecommunication system, or some other optical system or device. For brevity of the description, the electromagnetic energy may be referred to as light, light signal, laser light, laser beam, etc. (regardless of wavelength and regardless of whether in the visible spectrum or in some other wavelength range). The principles of the invention are to be understood as applicable for other DUT's, other electromagnetic energy, other electromagnetic energy sources, etc.
An exemplary way that optical components, such as passive optical components, may be tested is to direct a laser light into the optical component and to take appropriate measurements of light from the output of the optical component. Typically the laser light is provided by a TLS that is swept over a range of wavelengths. The sweeping may be continuous, e.g., as an analog sweep, or may be such as to produce light at a number of discrete wavelengths included in the range. Thus, the wavelength of the laser light changes over a period of time as the test is conducted. The optical characteristics of the optical component may be characterized at a (sometimes large) number of wavelengths.
To test many categories of optical components, especially those typically referred to as passive optical components or containing passive optical components, a light signal is provided at the input(s), and the light exiting the component is measured; the difference between the light at the input(s) and that at the output(s) is used to characterize the performance of the device. In one example of an ideal circumstance, the light signal would be provided as a signal that varies in an exact linear manner between wavelength and time (continuous sweep) or between wavelength and step number (stepped sweep); however, in many instances such ideal circumstance does not occur-one reason for nonlinearity is due to mode hop as is described herein.
To identify correctly the wavelength performance of an optical component, e.g., the performance of the optical component with respect to the wavelength of an incident laser beam from a TLS wavelength as the TLS is swept over a range of wavelengths (whether continuously in an analog fashion or at discrete wavelengths), the wavelength must be known very accurately, particularly at the time that measurements are taken with respect to the optical component. The TLS may be swept over such range once or repetitively, sometimes referred to as periodically or repeatedly swept over such range; and measurements can be taken of the optical component being tested using such TLS. Although the performance of tunable laser sources (TLSs) has been improving, there is an anomalous characteristic to the smooth wavelength change over time performance of tunable lasers, namely, “mode hop”, e.g., an instantaneous shift of wavelength, either forward or backward, by some number or amount, as was mentioned above. Occurrence of mode hop can detrimentally affect the accuracy of measurements made by an optical testing system and the characteristics obtained for the optical component being tested. Although efforts have been made to minimize the occurrence of mode hop, it does occur in at least some tunable laser systems. Some more expensive TLSs may be constructed and/or adjusted in an attempt to avoid mode hop over some range of wavelengths, but even these may encounter mode hop as the TLS ages and/or operating conditions, such as temperature and/or humidity, change.
With the above in mind, it would be desirable to be able to detect mode hop and to correct measurement data with respect to the mode hop. This ability may provide a number of advantages, such as, for example, improving the accuracy of the measurements and the characterizing of optical components; the ability to use less expensive laser sources that have mode hop but which still would be suitable for making desired measurements if mode hop wavelength error could be compensated, etc.; and the ability to use a TLS that may be relatively mode hop free over a narrow tuning range, but using the invention such TLS may be able to be used over a wider wavelength range for making measurements.
For background purposes, mode hop detection is generally needed to accurately measure optical power as a function of wavelength. There are different types of measurements that can be taken as a function of wavelength and present day equipment can be very accurate in terms of measuring optical power. In addition to optical power, wavelength is an important parameter to measure in order to allow telecom equipment to advance, e.g., by packing more and more channels into optical fibers. Therefore wavelength separation between channels requires finer and finer precision to accurately characterize devices. Devices may be characterized by a variety of parameters, including: optical power, insertion loss and various other power-related measurements. In addition to these measurements, it is also desired to calculate the above listed parameters as a function of wavelength.
Measurements of these parameters are generally made by using a tunable laser that sweeps across a range of wavelengths. Measurements are taken periodically over the range of wavelengths. An ideal tunable laser will sweep at a fixed rate and, for example, might start at 1525 nanometers in sweep to 1600 nanometers and perform the sweep at a fixed rate of perhaps 100 nanometers per second. This is ideal, but reality is that mechanisms inside the lasers will cause the wavelength to change not at a 100 nanometers per second, but at variable sweep rates during that sweep and that causes inaccuracies in wavelength. As a result, the actual wavelength as a function of time deviates from the ideal wavelength as function of time during that sweep.
To correct this problem various correction schemes have been developed to much more accurately determine wavelength as a function of time during that sweep. However, one of the non-idealities of tunable lasers is that as they sweep, the laser can mode hop. A mode hop changes the mode of the laser and is generally very difficult to correct. A typical mode hop might be approximately in the range of 15-45 picometers (pm). Mode hops affect measurement accuracy because it is generally desired to characterize devices with accuracy as small as one (1) pm. Therefore, mode hops pose a significant problem to obtaining the desired accuracy.